Elasticsearch: Fix URL creation and allowlist for `/_mapping` requests (#80970)
* Elasticsearch: Fix URL creation for mapping requests
* remove leading slash by default
* add comment for es route
* hardcode `_mapping`
* update doc
(cherry picked from commit 3d033839d7)
Co-authored-by: Sven Grossmann <sven.grossmann@grafana.com>
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The contributor documentation for Grafana's backend is in /contribute/backend/README.md.